1. Field of the Invention
The present invention relates to a focusing method in photolithography. In particular, the present invention concerns with an exposing system in which a mask pattern is successively transferred to predetermined regions of a wafer surface through a reducing projection lens in a step-and-repeat manner, and specifically relates to a method of focusing exposure light for each predetermined region before radiation of the exposure light in the exposing system
2. Description of the Background Art
A step-and-repeat photolithographic system with demagnification is a kind of projection exposure system for transferring a pattern provided at a mask onto a resist. This step-and-repeat photolithographic system with demagnification employs a step-and-repeat method and includes a mechanism executing this method.
In the step-and-repeat method, exposure is performed each time a wafer on a two-dimensionally movable X-Y stage is fed a constant distance in the process of transferring a mask pattern onto a resist. The exposing method by this step-and-repeat photolithographic system with demagnification will be described below.
FIG. 8 shows a conventional step-and-repeat photolithographic system with demagnification during an exposing process. Referring to FIG. 8, a light source 511 radiates light beams (g-ray or i-ray) of a mercury lamp to a glass mask (reticle) 515 through a condenser lens 513. The light beams passing through glass mask 515 are projected onto a photoresist on a wafer 50 through a reducing projection lens 501.
In the above step-and-repeat photolithographic system with demagnification, a region of about 15 mm.times.15 mm can be exposed at one time (one shot). Therefore, wafer 50 is exposed a shot each time wafer 50 is moved by an X-Y stage 502 provided for successively and automatically moving it in the X and Y directions.
Wafer 50 is vacuum-fixed on X-Y stage 502.
Although the above method in which the wafer is exposed a shot in accordance with the step-and-repeat manner provides a throughput lower than that in an entire wafer transferring method, it can provide the following advantages (1) and (2). (1) Since the projection area is reduced, it is possible to use a lens having a similar size but a larger numerical aperture (NA), so that it can provide a pattern having good resolution and size controllability. (2) Owing to the small projection area, distortion of an image by the lens can be suppressed, so that highly accurate positioning is allowed. In the process of exposing the wafer by this step-and-repeat photolithographic system with demagnification for forming a pattern of high resolution, an important factor exists in focusing of the exposure light prior to the exposure process. This focusing of exposure light is generally performed by an automatic focusing mechanism provided at the step-and-repeat photolithographic system with demagnification.
FIG. 9 schematically shows a structure of a conventional automatic focusing mechanism used in the step-and-repeat photolithographic system with demagnification. Referring to FIG. 9, the automatic focusing mechanism includes a light-emitting diode (LED) 503, condenser lens 504, projection slit 505, projection lens 506, receiving lens 507, oscillator 508, receiving slit 509 and detector 510.
Laser beams emitted from LED 503 pass through condenser lens 504, projection slit 505 and projection lens 506 and impinge on the surface of wafer 50. The laser beams reflected by the surface of wafer 50 pass through receiving lens 507 and are reflected at a predetermined angle by oscillator 508, and then the laser beams are received by detector 510 through receiving slit 509. Detector 510 detects the intensity of laser beams.
In the step of focusing the exposure light, wafer 50 is cover with a photoresist applied thereto.
Based on the intensity of laser beams detected by the detector, the system detects a level or vertical position of the surface portion of wafer 50 to which the laser beams are radiated. In order to attain the optimum focusing of the exposure light with respect to the level of the surface, X-Y stage 502 is vertically moved (i.e., in the z-direction) relatively to reducing projection lens 501. Thereby, the surface of wafer 50 receiving the laser beams is adjusted to occupy the optimum position with respect to reducing projection lens 501.
Then, a process of processing a wafer by one step-and-repeat photolithographic system with demagnification will be described below.
FIG. 10 is a flow chart schematically showing the process of processing the wafer by the step-and-repeat photolithographic system with demagnification. FIG. 11 is a flow chart schematically showing a level difference measuring step 131b and a focal position adjusting step 133 in a focusing step 130 in FIG. 10.
FIG. 12 is a schematic plan of the wafer showing an exposure region formed of a plurality of shot regions and a nonexposure region. FIG. 13 is a schematic plan showing a radiation position of laser beams in one shot.
Referring mainly to FIG. 10, a semiconductor wafer is transported into a step-and-repeat photolithographic system with demagnification (step 110). Thus, the wafer is laid on the X-Y stage and vacuum-fixed thereto. Alignment operation is performed for accurately aligning a mask pattern to be transferred with respect to the wafer (step 120). In this state, focusing with respect to the wafer is performed (step 130). This focusing step 130 includes level difference measuring step 131b and focal position adjusting step 133.
First, a predetermined number of shots are specified for measuring a level difference (.DELTA.F) at level difference measuring step 131b. More specifically, as shown in FIG. 12, an exposure region 57 on the wafer surface is formed of a plurality of shots 55 each defined by alternate long and short dash line. Shot 55 is a region to be exposed at a time in a step-and-repeat method by the step-and-repeat photolithographic system with demagnification. Shots 55 include chip regions 51, i.e., regions which will form chips, and dicing line regions 53 located between chip regions 51. Among these plurality of shots 55, a predetermined number of shots 55 are specified.
Referring mainly to FIG. 11, operation is performed to measure a level F.sub.A of a central portion (shot center) of one shot 55 among specified shots 55 (step 201). Thus, as shown in FIG. 13, laser beams 70 are radiated to a position 55a which can be deemed to be a substantially central position of shot 55, and level F.sub.A is detected based on the intensity of reflected beams of laser beams 70 as already described with reference to FIG. 9.
Then, the X-Y stage moves predetermined distances in the X and Y directions (step 203 in FIG. 11). Thereby, the radiation position of laser beams 70 are set at a reference position 55b which is in the same shot 55 as the central position 55a but is spaced by a predetermined distance therefrom. Reference position 55b is set at a substantially flat region 63 on the wafer surface in the shot 55.
In the chip region, surfaces of repetitive regions are flatter than that of the other region. Therefore, if the chip is, for example, a DRAM (Dynamic Random Access Memory), reference position 55b is set at a memory cell region MC which is the repetitive region.
Then, laser beams 70 are radiated to this reference position 55b to measure a level F.sub.B of position 55b (step 205 in FIG. 11).
Based on level F.sub.A of shot center 55a and level F.sub.B of reference position 55b, a difference .DELTA.F between these levels is obtained (step 207 in FIG. 11). Thus, as shown in FIG. 14, the level difference .DELTA.F is equal to a value (F.sub.B -F.sub.A) obtained by subtracting level F.sub.A of shot center 55a from level F.sub.B of reference position 55b forming the reference surface.
FIG. 14 is a fragmentary cross section of the wafer showing a structure including two chip regions 51 and dicing regions 53 in one shot 55. FIG. 14 also shows a structure in which shot center 55a is located at a dicing line region DL and reference position 55b is located at a memory cell region MC.
The shot center may be located at memory cell region MC as shown in FIG. 15. FIG. 15 shows a structure in which three chip regions 51 and dicing line regions 53 are included in one shot.
Operation is repeated to obtain level difference .DELTA.F in all the specified shots (step 209 in FIG. 11). After the level difference .DELTA.F is obtained in all the specified shots, an average value .DELTA.F.sub.ave of level differences .DELTA.F in the respective shots is calculated (step 211 in FIG. 11).
After the calculation of average.DELTA.F.sub.ave of the specified shots, operation is performed in the step-and-repeat manner as shown in FIG. 10, and more specifically, the focal position adjustment (step 133) and exposure with beams focused to this focal position (step 140) are performed for each shot.
Initially in the focal position adjusting step 133, a level F.sub.A1 of the shot center is measured in each shot to be exposed (step 221) as shown in FIG. 11.
Then, the optimum focusing position is determined based on level F.sub.A1 of the above shot center, average value .DELTA.F.sub.ave of level differences obtained at the above level difference measuring step 131b and focus offset F.sub.0 (step 223 in FIG. 11). Thus, as shown in FIG. 14, the optimum focusing position (surface) F.sub.s is determined by adding average value .DELTA.F.sub.ave and focus offset F.sub.0 to level F.sub.A1 (F.sub.A) of the shot center (F.sub.A1 +.DELTA.F.sub.ave +F.sub.0).
Then, the X-Y stage is vertically moved in the Z-direction relatively to the reducing projection lens for focusing the exposure light onto the optimum focusing position F.sub.s (step 225 in FIG. 11).
Referring mainly to FIG. 10, the exposure for one shot is then performed, so that the mask pattern is transferred onto the photoresist (step 140).
Then, the X-Y stage moves in the X and Y directions to enable exposure of the next shot (step 150). In this manner, the focal position adjusting step 133 and exposing step 140 are repeated for a plurality of shots (step 160).
After the mask pattern for all shots was transferred, the wafer is removed from the step-and-repeat photolithographic system with demagnification (step 170).
According to the conventional focusing method in the photolithography, level F.sub.B of reference position 55b (i.e., reference surface) is measured for obtaining the level difference .DELTA.F. Since the level F.sub.B of the reference surface represents a value which is utilized as the reference for obtaining the optimum focusing position, it is desired that the value is accurate. For this reason, the repetitive pattern region having a flatter surface than other region in the chip is employed as the reference surface. Thus, the repetitive pattern region has a relatively flat surface, so that the intensity of laser beams for measuring the level, which are radiated onto the repetitive pattern region and are reflected thereby, is hardly affected by the level difference of the surface, and thus the surface level can be accurately obtained.
However, even if the repetitive pattern region is used as the reference surface, failures, e.g., in the pattern configurations are caused if the degree of integration is increased. This will be described below.
(1) Sizes of a spot of laser beams 70 used for level measurement has a length L of 2 mm and a width W of 150 .mu.m on the wafer as shown in FIG. 13. Therefore, repetitive pattern region 63 receiving the laser beams must occupy a planar area of sizes not smaller than those of laser beam spot 70. If the repetitive pattern region 63 has the area of sizes not smaller than those of laser beam spot 70, laser beams 70a are applied to and reflected by only the repetitive pattern region 63 as shown in FIG. 16. Therefore, the level of repetitive pattern region 63 forming the reference surface can be measured accurately.
However, if the repetitive pattern region 63 did not have the area of sizes not smaller than those of laser beam spot 70, laser beam spot 70 would protrude from the repetitive pattern region 63 as shown in FIG. 17. In this case, laser beams 70a would be radiated also onto region 61 at different levels, so that the intensity of reflected laser beams 70a would vary, and thus the level would not be measured accurately.
(2) Some kinds of ICs (Integrated Circuits) such as logic LSIs (e.g., ASICs (Application Specific Integrated Circuits)) do not have a region such as a repetitive region which cannot be deemed as a flat region. In this case, it is impossible to measure accurately a level of the region to be utilized as the reference surface by radiation of laser beams.
Under the conventional design rule up to about 0.8 .mu.m, the exposure device has a sufficient margin with respect to the design rule, and has a sufficiently large focal depth. For the logic LSIs such as ASICs, the focus is not set individually for respective types of circuits, but a uniform focus setting is effected on all the products designed under the same rule. However, elements have now been miniaturized to a higher extent in accordance with high integration, and thus some kinds of elements are now produced under the design rule not more than 0.6 .mu.m, so that it is demanded to set the focal depth under severer conditions.
Under the above design rule set for further miniaturization, if the level of reference surface were not measured as described in the above cases (1) and (2), it would be impossible to perform optimum focusing of the exposure light. This would cause failure in pattern configuration, e.g., during formation of the pattern.